Method for seamlessly changing power modes in an ADSL system

ABSTRACT

A DMT system and method with the capability to adapt the system bit rate on-line in a seamless manner. The DMT system provides a robust and fast protocol for completing this seamless rate adaptation. The DMT system also provides a framing and encoding method with reduced overhead compared to conventional DMT systems. The DMT system and method provide seamless rate adaptation with the provision of different power levels. This framing and encoding method enables a system with seamless rate adaptation capability. The system and method of the invention can be implemented in hardware, or alternatively in a combination of hardware and software.

RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. provisionalapplication Ser. No. 60/124,222, filed Mar. 12, 1999, entitled “SeamlessRate Adaptive (SRA) ADSL System”, U.S. provisional application Ser. No.60/161,115, filed Oct. 22, 1999, entitled “Multicarrier System withStored Application Profiles”, and U.S. provisional application Ser. No.60/171,081, filed Jan. 19, 2000, entitled “Seamless Rate Adaptive (SRA)Multicarrier Modulation System and Protocols,” which copendingprovisional applications are incorporated herein by reference in theirentirety.

FIELD OF THE INVENTION

This invention relates generally to communication systems and methodsusing multicarrier modulation. More particularly, the invention relatesto communication multicarrier systems and methods using rate adaptivemulticarrier modulation.

BACKGROUND OF THE INVENTION

Multicarrier modulation (or Discrete Multitone Modulation (DMT)) is atransmission method that is being widely used for communication overdifficult media Multicarrier modulation divides the transmissionfrequency band into multiple subchannels (carriers), with each carrierindividually modulating a bit or a collection of bits. A transmittermodulates an input data stream containing information bits with one ormore carriers and transmits the modulated information. A receiverdemodulates all the carriers in order to recover the transmittedinformation bits as an output data stream.

Multicarrier modulation has many advantages over single carriermodulation. These advantages include, for example, a higher immunity toimpulse noise, a lower complexity equalization requirement in thepresence of multipath, a higher immunity to narrow band interference, ahigher data rate and bandwidth flexibility. Multicarrier modulation isbeing used in many applications to obtain these advantages, as well asfor other reasons. Applications include Asymmetric Digital SubscriberLine (ADSL) systems, Wireless LAN systems, Power Line communicationssystems, and other applications. ITU standards G.992.1 and G.992.2 andthe ANSI T1.413 standard specify standard implementations for ADSLtransceivers that use multicarrier modulation.

The block diagram 100 for a standard compliant ADSL DMT transmitterknown in the art is shown in FIG. 1. FIG. 1 shows three layers: theModulation layer 110, the Framer/FEC layer 120, and the ATM TC layer140, which are described below.

The Modulation layer 110 provides functionality associated with DMTmodulation. DMT modulation is implemented using an Inverse DiscreteFourier Transform (IDFT) 112. The IDFT 112 modulates bits from theQuadrature Amplitude Modulation (QAM) 114 encoder into the multicarriersubchannels. ADSL multicarrier transceivers modulate a number of bits oneach subchannel, the number of bits depending on the Signal to NoiseRatio (SNR) of that subchannel and the Bit Error Rate (BER) requirementof the link. For example, if the required BER is 1×10⁻⁷ (i.e., one bitin ten million is received in error on average) and the SNR of aparticular subchannel is 21.5 dB, then that subchannel can modulate 4bits, since 21.5 dB is the required SNR to transmit 4 QAM bits with a1×10⁻⁷ BER. Other subchannels can have a different SNR and therefore mayhave a different number of bits allocated to them at the same BER. TheITU and ANSI ADSL standards allow up to 15 bits to be modulated on onecarrier.

A table that specifies how many bits are allocated to each subchannelfor modulation in one DMT symbol is called a Bit Allocation Table (BAT).A DMT symbol is the collection of analog samples generated at the outputof the IDFT by modulating the carriers with bits according to the BAT.The BAT is the main parameter used in the Modulation layer 110 ofFIG. 1. The BAT is used by the QAM 114 and IDFT 112 blocks for encodingand modulation Table 1 shows an example of a BAT for a DMT system with16 subchannels. TABLE 1 Example of BAT for multicarrier system with 16subchannels Subchannel Bits per Number Subchannel  1 5  2 9  3 3  4 2  54  6 0  7 5  8 7  9 8 10 3 11 0 12 5 13 6 14 8 15 4 16 3 Total bits 80 Per DMT symbol

In ADSL systems the DMT symbol rate is approximately 4 kHz. This meansthat a new DMT symbol modulating a new set of bits, using the modulationBAT, is transmitted every 250 microseconds. If the BAT in table 1, whichspecifies 80 bits modulated in one DMT symbol, were used at a 4 kHz DMTsymbol rate the bit rate of the system would be 4000*80=320 kilobits persecond (kpbs). The BAT determines the data rate of the system and isdependent on the transmission channel characteristics, i.e. the SNR ofeach subchannel in the multicarrier system. A channel with low noise(high SNR on each subchannel) will have many bits modulated on each DMTcarrier and will thus have a high bit rate. If the channel conditionsare poor, the SNR will be low and the bits modulated on each carrierwill be few, resulting in a low system bit rate. As can be seen in Table1, some subchannels may actually modulate zero bits. An example is thecase when a narrow band interferer (such as AM broadcast radio) ispresent at a subchannel's frequency and causes the SNR in thatsubchannel to be too low to carry any information bits.

The ATM TC layer 140 includes an Asynchronous Transfer Mode TransmissionConvergence (ATM TC) block 142 that transforms bits and bytes in cellsinto frames.

The next layer in an ADSL system is the Frame/FEC layer 120, whichprovides functionality associated with preparing a stream of bits formodulation, as shown in FIG. 1. This layer contains the Interleaving(INT) block 122, the Forward Error Correction (FEC) block 124, thescrambler (SCR) block 126, the Cyclic Redundancy Check (CRC) block 128and the ADSL Framer block 130. Interleaving and FEC coding provideimpulse noise immunity and a coding gain. The FEC 124 in the standardADSL system is a Reed-Solomon (R-S) code. The scrambler 126 is used torandomize the data bits. The CRC 128 is used to provide error detectionat the receiver. The ADSL Framer 130 frames the received bits from theATM framer 142. The ADSL framer 130 also inserts and extracts overheadbits from module 132 for modem to modem overhead communication channels(known as EOC and AOC channels in the ADSL standards).

The key parameters in the Framer/FEC layer 120 are the size of the R-Scodeword, the size (depth) of the interleaver (measured in number of R-Scodewords) and the size of the ADSL frame. As examples, a typical sizefor an R-S codeword may be 216 bytes, a typical size for interleaverdepth may be 64 codewords, and the typical size of the ADSL frame may be200 bytes. It is also possible to have an interleaving depth equal toone, which is equivalent to no interleaving. In order to recover thedigital signal that was originally prepared for transmission using atransmitter as discussed above, it is necessary to deinterleave thecodewords by using a deinterleaver that performs the inverse process tothat of the interleaver, with the same depth parameter. In the currentADSL standards there is a specific relationship between all of theseparameters in a DMT system. Specifically, the BAT size, N_(BAT) (totalnumber of bits in a DMT symbol) is fixed to be an integer divisor of theR-S codeword size, N_(FEC), as expressed in equation (1):N _(FEC) =S×N _(BAT), where S is a positive integer greater than 0.  (1)

This constraint can also be expressed as: One R-S codeword contains aninteger number of DMT symbols. The R-S codeword contains data bytes andparity (checkbytes). The checkbytes are overhead bytes that are added bythe R-S encoder and are used by the R-S decoder to detect and correctbit errors. There are R checkbytes in a R-S codeword. Typically, thenumber of checkbytes is a small percentage of the overall codeword size,e.g., 8%. Most channel coding methods are characterized by their codinggain, which is defined as the system performance improvement (in dB)provided by the code when compared to an uncoded system. The codinggain, of the R-S codeword depends on the number of checkbytes and theR-S codeword size. A large R-S codeword (greater than 200 bytes in a DMTADSL system) along with a 16 checkbytes (8% of 200 bytes) will provideclose to the maximum coding gain of 4 dB. If the codeword size issmaller and/or the percentage of checkbyte overhead is high (e.g. >30%)the coding gain may be very small or even negative. In general, it isbest to have the ADSL system operating with the largest possible R-Scodeword (the maximum possible is 255 bytes) and approximately 8%redundancy.

There is also a specific relationship between the number of bytes in anADSL frame, N_(FRAME), and the R-S codeword size, N_(FEC) that isexpressed in equation (2):N _(FEC) =S×N _(FRAME) +R; where R is the number of R-S checkbytes in acodeword and  (2)

S is the same positive integer in Equation (1).

It is apparent from equating the right-hand sides of equations (1) and(2) that the relationship expressed in equation (3) resultsN _(BAT) =N _(FRAME) +R/S.  (3)The ADSL standard requires that the ratio (R/S) is an integer, i.e.there is an integer number of R-S checkbytes in every DMT-symbol(N_(BAT)). As described above, ADSL frames contain overhead bytes (notpart of the payload) that are used for modem to modem communications. Abyte in an ADSL frame that is used for the overhead channel cannot beused for the actual user data communication, and therefore the user datarate decreases accordingly. The information content and format of thesechannels is described in the ITU and ANSI standards. There are severalflaming modes defined in ADSL standards. Depending on the framing mode,there are more or fewer overhead bytes in one ADSL flame. For example,standard Framing Mode 3 has 1 overhead byte per ADSL frame.

Equations (1), (2) and (3) demonstrate that the parameter restrictionsimposed by the standards result in the following conditions:

1. All DMT symbols have a fixed number of overhead framing bytes thatare added at the ADSL framer. For example, in framing mode #3 there is 1overhead framing byte per DMT symbol.

2. There is a minimum of 1 R-S checkbyte per DMT symbol.

3. The maximum number of checkbytes according to ITU Standard G.992-2(8) and ITU Standards G.992.2 and T1.413 (16) limits the maximumcodeword size to 8*N_(BAT) for G.992.2, and to 16*N_(BAT) for 6.992.1and T1.413.

4. An ADSL modem cannot change the number of bits in a DMT symbol(N_(BAT)) without making the appropriate changes to the number of bytesin a R-S codeword (N_(FEC)) and an ADSL frame (N_(FRAME)).

The above four restrictions cause performance limitations in currentADSL systems.

In particular, because of condition #1 every DMT symbol has a fixednumber of overhead framing bytes. This is a problem when the data rateis low and the overhead framing bytes consume a large percentage of thepossible throughput resulting in a lower payload. For example, if thedate rate supported by the line is 6.144 Mbps, this will result in a DMTsymbol with about 192 bytes per symbol (192*8*4000=6144000 bps). In thiscase, one overhead framing byte would consume 1/1 92 or about 0.5% ofthe available throughout. But if the date rate is 128 kbps or 4 bytesper symbol the overhead framing byte will consume ¼ or 25% of theavailable throughput. Clearly this is undesirable.

Condition #2 will cause the same problems as condition # 1. In thiscase, the overhead framing byte is replaced by the R-S checkbyte.

Condition #3 will not allow the construction of large codewords when thedata rate is low. R-S codewords in ADSL can have a maximum of 255 bytes.The maximum coding gain is achieved when the codeword size is near themaximum 255 bytes. When the data rate is low, e.g. 128 kbps or 4 bytesper symbol, the maximum codeword size will be 8*4=32 bytes for G.992.2systems and 16*4=64 bytes for G.992.1 and T1.413 systems. In this casethe coding gain will be substantially lower than for large codewordsapproaching 255 bytes.

In general, if the data rate is low, e.g. 128 kbps or 4 byte per symbol,the above conditions will result in 1 byte being used for overheadframing, and 1 byte being consumed by a R-S checkbyte. Therefore 50% ofthe available throughput will not be used for payload and the R-Scodeword size will be at most 64 bytes, resulting in negligible codinggain.

Condition #4 effects the ability of the modem to adapt its transmissionparameters on-line in a dynamic manner.

G.992-1 and T1.413 specify a mechanism to do on-line rate adaptation,called Dynamic Rate Adaptation (DRA), but it is clearly stated in thesestandards that the change in data rate will not be seamless. In generalcurrent ADSL DMT modems use Bit Swapping and dynamic rate adaptation(DRA) as methods for on-line adaptation to channel changes. Bit Swappingis specified in the ITU and ANSI standards as method for modifying thenumber of bits allocated to a particular. Bit Swapping is seamless,i.e., it does not result in an interruption in data transmission andreception. But, Bit Swapping does not allow the changing of data rates.Bit Swapping only allows the changing the number of bits allocated tocarriers while maintaining the same data rate. This is equivalent tochanging the entries in the BAT table without allowing the total numberof bits (N_(BAT)) in the BAT to increase or decrease.

DRA enables a change in data rate, but is not seamless. DRA is also veryslow because it requires the modem located in the Central Office (CO) tomake the final decision on the data rate configuration. This model, with(the CO being the master), is common among ADSL modems that are designedto provide a service offered by the telephone company, and controlled bythe telephone company.

Both Bit Swapping and DRA use a specific protocol that is specified inANSI T1.413, G.992-1 and G.992.2 for negotiating the change. Thisprotocol negotiates the parameters using messages that are sent via anAOC channel, which is an embedded channel. This protocol is sensitive toimpulse noise and high noise levels. If the messages are corrupted thetransmitter and receiver can enter a state where they are usingdifferent transmission parameters (e.g., BAT, data rate, R-S codewordlength, interleaver depth, etc). When two communication modems enter astate of mismatched transmission parameters, data will be received inerror and the modems will eventually be required to take drasticmeasures (such as full reinitialization) in order to restore error freetransmission. Drastic measures such as full reinitialization will resultin the service being dropped for approximately 10 seconds, which is thetime required for a standards compliant ADSL modem to complete a fullinitialization.

A transceiver has both a transmitter and a receiver. The receiverincludes the receiver equivalent blocks of the transmitter shown inFIG. 1. The receiver has modules that include a decoder, a deinterleaverand a demodulator. In operation, the receiver accepts a signal in analogform that was transmitted by a transmitter, optionally amplifies thesignal in an amplifier, filters the signal to remove noise componentsand to separate the signal from other frequencies, converts the analogsignal to a digital signal through the use of an analog to digitalconverter, demodulates the signal to generate the received bits streamfrom the carrier subchannels by the use of a demodulator, deinterleavesthe bits stream by the use of a deinterleaver, performs the FEC decodingto correct errors in the bit stream by use of an FEC decoder,descrambles the bit stream by use of a descrambler, and detects biterrors in the bit stream by use of a CRC. Various semiconductor chipmanufacturers supply hardware and software that can perform thefunctions of a transmitter or a receiver, or both.

It is therefore apparent that there is a need for an improved DMTtransmission system. It is therefore a principle object of the inventionto provide an improved DMT transmission system that overcomes theproblems discussed above.

SUMMARY OF THE INVENTION

According to the principles of the invention, ADSL DMT systems andmethods are provided that change transmission bit rates in a seamlessmanner during operation. The ADSL DMT systems and methods operateaccording to protocols that allow the seamless change of transmissionbit rates during operation to be initiated by either the transmitter orthe receiver. The ADSL DMT systems and methods provide for seamlesschanges of transmission bit rates during operation that changetransmission bit rates between power levels that range from full powerto low power.

In one aspect, the invention relates to a method for seamlessly enteringa second power mode from a first power mode. The method uses amulticarrier transmission system that includes a transmitter and areceiver. The transmitter and receiver use a first bit allocation tableto transmit a plurality of codewords at a first transmission bit rate ina first power mode. The plurality of codewords have a specified codewordsize and include a specified number of parity bits for forward errorcorrection, and a specified interleaving parameter for interleaving theplurality of codewords. The method involves storing a second bitallocation table at the receiver and at the transmitter for transmittingcodewords at a second transmission in the second power mode. The methodincludes synchronizing use of the second bit allocation table betweenthe transmitter and receiver, and entering the second power mode byusing the second bit allocation table to transmit codewords In order toachieve a seamless change in power mode, the specified interleavingparameter, the specified codeword size, and the specified number ofparity bits for forward error correction used to transmit codewords inthe first power mode are also used to transmit codewords in the secondpower mode.

In one embodiment, the synchronizing includes sending a flag signal. Inanother embodiment, the flag signal is a predefined signal. In a furtherembodiment, the predefined signal is a sync symbol with a predefinedphase shift. In a still further embodiment, the predefined signal is aninverted sync symbol. In another embodiment, the transmitter transmitsthe flag signal to the receiver. In a different embodiment, the receivertransmits the flag signal to the transmitter. In another embodiment, thesecond power mode is a low power mode.

In another embodiment, the method further involves allocating zero bitsto carrier signals to achieve a transmission bit rate of approximatelyzero kilobits per second in the low power mode. In another embodiment,the method further includes transmitting a pilot tone for timingrecovery when operating in the low power mode. In still anotherembodiment, the method further comprises periodically transmitting async symbol when operating in the low power mode.

In still another embodiment, the method further includes using the firstbit allocation table for transmitting a plurality of DMT symbols in thefirst power mode and switching to the second bit allocation table fortransmitting the plurality of the DMT symbols in the second power mode.The second bit allocation table is used for transmission starting with apredetermined one of the DMT symbols that follows the transmission ofthe flag signal. In another embodiment, the predetermined DMT symbol isthe first DMT symbol that follows the transmission of the flag signal.

In another embodiment, the second power mode is a full power mode. Instill another embodiment, the first power mode is a full power mode, andthe second power mode is a low power mode. In another embodiment, thefirst power mode is a low power mode, and the second power mode is afull power mode.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention can be better understood withreference to the drawings described below. The drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention.

FIG. 1 is a block diagram for a standard compliant ADSL DMT transmitterknown in the prior art.

FIG. 2 is an exemplary embodiment of an ADSL frame and R-S codeword.

FIG. 3 is a block diagram for a dual latency ADSL DMT transmitter.

FIG. 4 is a flow chart that depicts an embodiment of a process in whicha Normal Seamless Rate Adaptive (NSRA) transmission bit rate change isinitiated by a receiver according to the principles of the invention.

FIG. 5 is a flow chart that depicts an embodiment of a process in whicha Normal Seamless Rate Adaptive (NSRA) transmission bit rate change isinitiated by a transmitter according to the principles of the invention.

FIG. 6 is a flow chart that depicts an embodiment of a process in whicha Fast Seamless Rate Adaptive (FSRA) transmission bit rate change isinitiated by a receiver according to the principles of the invention.

FIG. 7 is a flow chart that depicts an embodiment of a process in whicha Fast Seamless Rate Adaptive (FSRA) transmission bit rate change isinitiated by a transmitter according to the principles of the invention.

DETAILED DESCRIPTION

The principles of the invention may be employed using transceivers thatinclude a transmitter, such as that described in FIG. 1 above, and areceiver. In general terms, an ADSL system includes both a transmitterand a receiver for each communication in a particular direction. In thediscussion that follows, an ADSL DMT transmitter accepts digital inputand transmits analog output over a transmission line, which can be atwisted wire pair, for example. The transmission can also occur over amedium that includes other kinds of wires, fiber optic cable, and/orwireless connections. In order to utilize the transmitted signal, asecond transceiver at the remote end of the transmission line includes areceiver that converts the received analog signal into a digital datastream for use by devices such as computers or digital televisions, forexample. For bidirectional communication using a pair of transceivers,each transceiver includes a transmitter that sends information to thereceiver of the other member of the pair, and a receiver that acceptsinformation transmitted by the transmitter of the other member of thepair.

This invention describes a DMT system with the capability to adapt thesystem bit rate on-line in a seamless manner. The DMT system alsoprovides a robust and fast protocol for completing this seamless rateadaptation. The DMT system also provides a framing and encoding methodwith reduced overhead compared to conventional DMT systems. This newframing and encoding method also enables a system with seamless rateadaptation capability.

It may be desirable to change the modem data rate after training due toa change in the channel characteristics or because the applicationrunning over ADSL has changed. Examples of changing channelcharacteristics include changes in the noise on the line, changes in thecrosstalk from other services in the bundle or on the same line, changesin the levels and presence of Radio Frequency Interference ingress,changes in the line impedance due to temperature changes, changes in thestate of equipment on the line (e.g. a phone going from on-hook to offhook, or vice versa), and the like. Examples of changes in applicationsinclude power down modes for a PC, a user changing from Internetbrowsing to two-way video conferencing, a user changing from internetbrowsing to voice over DSL with or without internet browsing, and thelike. It is often desirable or required to change the data rate of themodem. It is highly desirable that this data rate change occurs in a“seamless” manner, i.e., without data bit errors or an interruption inservice. However DMT ADSL modems specified in the prior art standardsare not capable of performing seamless data rate adaptation.

Condition #4 described previously, does not allow the size of the BAT tochange without modifying the R-S coding, interleaving and framingparameters. If the BAT, and N_(BAT), could be modified during operation,i.e., if more or fewer bits were allocated to carriers in a DMT symbol,the data rate could be changed. Condition #4 requires that when thenumber of bits N_(BAT) in the BAT changes the size of the R-S codeword(and therefore interleaving parameters) must also be modified. Modifyingthe interleaving and coding parameters on-line requires re-initializingthe interleaver. Re-initialization of the interleaver always results ina “flushing” of the interleave memory. This flushing of memory willresult in data errors and the transition will not be seamless.

In order to allow a DMT ADSL transmission system to change data rateseamlessly, the invention relates to the following:

1. a more efficient method for framing and encoding the data, thatresults in less overhead data bits per DMT symbol, thereby increasingthe user bit rate;

2. a new ADSL system with the ability to dynamically adapt the data rateon-line (e.g., during operation) in a seamless manner; and

3. a new robust and fast protocol for completing such a seamless rateadaptation, so a data rate change can occur successfully even in thepresence of high levels of noise.

Constant Percentage Overhead Framing

In one embodiment, a framing method is described that decreases theoverhead (non-payload) data in DMT ADSL systems. FIG. 2 shows a diagram200 representative of an ADSL frame and R-S codeword that includes atleast one framing overhead byte 202, one or more payload bytes 204, andone or more checkbytes 206. This framing method also enables seamlessrate adaptation. As described above current ADSL systems placerestrictions and requirements on the ADSL frames, R-S codewords and DMTsymbols. A system implemented according to the principles of theinvention de-couples ADSL frames and R-S codewords from DMT symbols.This decoupling results in a system that has lower overhead data per DMTsymbol and also can complete online rate adaptations in a seamlessmanner. According to the principles of the invention, ADSL frames andR-S codewords are constructed to have the same length and to be aligned(see FIG. 2). The R-S codeword is made sufficiently large enough tomaximize the coding gain. The size of the R-S codeword (and thereforeADSL frame) can be negotiated at startup or fixed in advance. A fixednumber of R-S checkbytes and overhead framing bytes are included in anADSL frame. These parameters can also be negotiated at startup or fixedin advance.

Unlike DMT symbols of the prior art, DMT symbols produced in accordancewith the principles of the invention are not aligned with ADSL framesand R-S codewords. Also the number of bits in a DMT symbol dependssolely on the data rate requirements and configurations and isde-coupled from the R-S codeword size, the interleaver depth and theADSL frame size. The number of bits in a DMT symbol dictates the datarate of the modem independently of the other framing, coding orinterleaving restrictions. Since overhead bytes are added at the ADSLframe layer, a DMT symbol does not necessarily contain a fixed number ofoverhead bytes. As the data rate gets lower, for example 128 kbps, theoverhead data remains low. In particular, this framing method assigns afixed percentage of overhead data to the data stream, rather than afixed number of overhead bytes. This percentage does not change when thedata rate of the modem changes, (as is the case with current ADSLmodems). Consider the following examples of conventional standardcompliant framing methods.

PRIOR ART EXAMPLE #1

The line capacity is 192 bytes per DMT symbol (6.144 Mbps) The codewordsize is 192, which includes 16 checkbytes and 1 overhead framing byte,(assuming ANSI T1.413 framing mode #3). The total framing overhead(i.e., checkbytes+overhead framing bytes) per DMT symbol is 16+1=17, andtherefore the framing overhead is 17/192=8.8% of the availablethroughput. In this case the framing overhead is reasonable.

PRIOR ART EXAMPLE #2

The line capacity is 4 bytes (128 kbps). The codeword is constructedfrom 16 DMT symbols and is 16*4=64 bytes. There are 16 R-S checkbytes (1checkbyte per DMT symbol) and there is 1 overhead framing byte (assumingANSI T1.413 framing mode #3). The total framing overhead(checkbytes+overhead framing bytes) per DMT symbol is 1+1=2 bytes, andtherefore the framing overhead is 2/4=50% of the available throughput.This is highly inefficient.

Examples of embodiments of the framing method of the invention providethe following results, called the Constant Percentage Overhead Method:

EXAMPLE # 1

This is exactly the same as the standard compliant training example(Prior Art Example #1) given above. Codeword sizes, DMT symbol sizes andoverhead are the same. Therefore the framing overhead is 17/192=8.8% ofthe available throughput as well.

EXAMPLE #2

The line capacity is 4 bytes (128 kbps). The codeword is constructedindependently of the DMT symbol and therefore could be set to 192 bytes,(as an example). This is also the size to of the ADSL frame. We use 16R-S bytes and 1 overhead framing byte per codeword or ADSL frame. Thereare 192/4=48 DMT symbols in 1 codeword. The total overhead(checkbytes+overhead framing bytes) per 48 DMT symbols is 1+16=17 bytesor 17/48=0.35 bytes per 1 DMT symbol. The framing overhead is0.35/4=8.8% of the available throughput.

From examples #1 and #2, it is apparent that the principles of theinvention provide a method to achieve a framing overhead that is aconstant percentage of the available throughput regardless of the datarate or the line capacity. In these examples, the framing overhead was8.8% for both 6 Mbps and 128 kbps.

Seamless Rate Adaptation (SRA) System

Another benefit of the framing method described in this invention isthat it enables seamless on-line rate adaptation. Seamless RateAdaptation (SRA) is accomplished by changing the DMT symbol BAT, i.e.the number of bits allocated to each subchannel in the Multicarriersystem. As shown above modifying the BAT changes the number of bits perDMT symbol and results in a change in the data bit rate of the system.In one embodiment, the DMT symbol size is changed without modifying anyof the RS coding, interleaving and framing parameters. This is possiblebecause the Constant Percentage Overhead framing method described aboveremoves the restrictions imposed by the prior art on the relationbetween DMT symbols and R-S codewords or ADSL frames. Since the R-Scoding and interleaving parameters do not change, interleaver flushingand other problems associated with changing the parameters associatedwith these functions do not occur. The transceiver can adapt the datarate without errors or service interruption. The only parameter thatneeds to be adapted is the BAT.

The BAT needs to be changed at the transmitter and the receiver atexactly the same time, i.e., on exactly the same DMT symbol. If thetransmitter starts using the new BAT for transmission before thereceiver does, the data is not demodulated correctly and bit errorsoccur. Also, if the receiver changes to a new BAT before the transmitterdoes, the same errors can occur. For this reason the transition to theuse of the new BAT for transmission and reception needs to besynchronized at the transmitter and the receiver. In-one embodiment, theprinciples of the invention provide a protocol that enables thesynchronized transition to the use of the new BAT.

It is also very important that this protocol is very robust in thepresence of channel noise. For example, if the protocol fails and thereceiver does not switch to the new BAT at the same time as thetransmitter, then bit errors occur and the transition is not seamless.Furthermore, if the transmitter and receiver are using different BATs,it is very difficult to re-establish an error free link withoutperforming a re-initialization of the connection, which results in aninterruption in service of up to 10 seconds.

It is also important that the transition between BATs occurs veryquickly, because the need to operate at a new data rate is usuallyinstantaneous. As an example, at a constant data rate a sudden decreasein the channel SNR will increase the number of bits received in error. Achange in data rate is required because of the reception of many bits inerror. In this situation, it is desirable to change the data rate assoon as possible to get out of the state of receiving bits in error. Asanother example, a change in the applications being transported over theADSL link can require a change in the data rate. For example if one useris browsing the Internet and then another user wishes to make a voicecall over the flow of data bits using the Voice over DSL capability ofthe ADSL connection, it is necessary to quickly change the data rate ofthe system to accommodate the telephone call in addition to the existingtraffic.

It is apparent from these requirements that it is necessary for the SRAprotocol to provide:

a. a method for synchronizing the transmitter and receiver transition tothe new BAT;

b. robust transition to the new data rate; and

c. fast transition to the new data rate.

The principles of the invention provide two protocols that satisfy theserequirements for seamless rate adaptation. The protocols are called theNormal SRA protocol and the Fast SRA protocol.

Normal SRA (NSRA) protocol

Either the transmitter or the receiver can initiate the Normal SRA(NSRA) protocol.

Receiver Initiated NSRA

The receiver initiated NSRA involves the following steps:

1. During initialization the transmitter and the receiver exchangeinformation describing their maximum and minimum data rate capabilities.This corresponds to the maximum and minimum number of bits per DMTsymbol.

2. During operation, the receiver determines that the data rate shouldbe increased or decreased.

3. If the new data rate is within the transmitter's rate capabilities,the receiver proceeds to step 4.

4. The receiver sends the new BAT and the new data rate to thetransmitter using the AOC or EOC channel. This corresponds to “NSRARequest” by the receiver.

5. The transmitter receives the “NSRA Request”.

6. The transmitter uses an inverted synchronization (sync) symbol as aflag to signal the receiver that the new BAT is going to be used. Thenew BAT is used for transmission on the first frame, or a finite numberof frames, following the inverted sync symbol. The inverted sync symboloperates as a rate adaptation “SRA Go” message sent by the transmitter.

7. The receiver detects the inverted sync symbol (“SRA Go”) and the newBAT is used for reception on the first frame, or a finite number offrames, following the inverted sync symbol.

FIG. 4 shows a flow chart 400 depicting an embodiment of a process inwhich a Normal Seamless Rate Adaptive (NSRA) transmission bit ratechange is initiated by a receiver according to the principles of theinvention. In FIG. 4, the steps described in action boxes 410 through470 correspond to the preceding discussion.

Transmitter Initiated NSRA

The transmitter initiated NSRA involves the following steps:

1. During initialization the transmitter and the receiver exchangeinformation describing their maximum and minimum capabilities regardingdata rate. This corresponds to the maximum and minimum number of bitsper DMT symbol.

2. The transmitter determines that the data rate should be increased ordecreased.

3. If the new desired data rate is within the receiver's rate capabilitythen the transmitter proceeds to step 4.

4. The transmitter sends to the receiver the new desired data rate usingthe EOC or AOC channel. This is an “NSRA Request” message.

5. The receiver receives the NSRA request message. If the channel cansupport the new data rate then the receiver proceeds to step 6. If thechannel can not support the new data rate then the receiver sends an“SRA Deny’ message back to the transmitter using the EOC or AOC channel.

6. The receiver sends the new BAT to the transmitter using the AOC orEOC channel based on the new data rate. This corresponds to an “NSRAGrant” request by the receiver.

7. The transmitter receives the “NSRA Grant”.

8. The transmitter uses an inverted sync symbol as a flag to signal thereceiver that the new BAT is going to be used. The new table is used fortransmission on the first frame, or a finite number of frames, followingthe inverted sync symbol. The inverted sync signal operates as a rateadaptation “SRA Go” message sent by the transmitter.

9. The receiver detects the inverted sync symbol (“SRA Go”) and the newtable is used for reception on the first frame, or a finite number offrames, following the inverted sync symbol.

FIG. 5 shows a flow chart 500 depicting an embodiment of a process inwhich a Normal Seamless Rate Adaptive (NSRA) transmission bit ratechange is initiated by a transmitter according to the principles of theinvention. In FIG. 5, the steps described in action boxes 510 to through590 correspond to the preceding discussion.

The rate adaptation only involves changing the number of bits in a DMTsymbol by changing the BAT, and not the R-S codeword size, interleaverdepth, or the ADSL frame size. This can be done without any interruptionin data flow or introduction of data errors.

This protocol of the invention is faster than conventional rateadaptation methods because it does not require an extended handshakebetween the transmitter and the receiver in order to approve the newtransmission parameters and rates. No extended handshake is neededbecause the data rate capabilities are known in advance and negotiatedduring startup. Also, the other parameters (such as R-S codeword length,interleaver depth, etc) are not changed during the data rate changeusing the new framing method.

This protocol of the invention is more robust than conventional rateadaptation techniques because it does not use the EOC or AOC channel tosend the “SRA Go” message for synchronizing the transition to the newdata rate. In conventional rate adaptation techniques, messages sentover the EOC and AOC channel can easily become corrupted by noise on theline. These overhead channels are multiplexed into the data stream atthe framer and therefore are transmitted with quadrature amplitudemodulation over a finite number of DMT subchannels. Impulse noise orother noise that occurs on the line can easily cause bit errors in theAOC channel message; the message can be lost. If the “SRA Go” message iscorrupted and not received by the receiver, then the receiver does notknow if the SRA request was granted or not. The transmitter, on theother hand, assumes the “SRA Go” message was received and switches tothe new data rate and transmission parameters. The receiver, which didnot receive the grant message, does not know when to switch to the newrate. The modems are unsynchronized and data errors occur.

The protocol of the invention is robust because, unlike conventionalrate adaptation techniques, the “SRA Go” message is not sent via an EOCor AOC message that can easily be corrupted. Instead the grant of therate adaptation request is communicated via an inverted sync symbol. Thesync symbol is defined in the ANSI and ITU standards as fixed non-datacarrying DMT symbol that is transmitted every 69 symbols. The syncsymbol is constructed by modulating all the DMT carriers with apredefined PN sequence using basic QPSK (2 bit QAM) modulation. Thissignal, which is used throughout the modem initialization process, hasspecial autocorrelation properties that make possible the detection ofthe sync symbol and the inverted sync symbol even in highly noisyenvironments. An inverted sync symbol is a sync symbol in which thephase information in the QAM signal is shifted by 180 degrees. Otherphase shifts (other than 180 degrees) of the sync symbol can be used aswell for the SRA Go message. Using the sync symbol for the “SRA Go”message makes the rate adaptation protocol very robust even in thenoisiest environments.

Fast SRA (FSRA) Protocol Using Stored BATs

The Fast SRA (FSRA) protocol seamlessly changes the data rate on theline faster than the NSRA protocol. This is important for certainapplications that are activated and de-activated instantaneously overtime or when sudden channel changes occur. In the FSRA protocol, “storedBATs” are used to speed up the SRA handshake and enable quick changes indata rate. Unlike profiles used in G.992.2, the stored BAT does notcontain the R-S coding and interleaving parameters since theseparameters are not effected when a rate change occurs using constantpercentage overhead framing.

The BATs are exchanged using NSRA described in the previous section.After the one time NSRA is complete, and a BAT that is based on thatparticular channel condition or application condition is stored by bothtransceivers, the FSRA protocol can use the stored BAT to complete faston-line rate adaptation. Stored BATs are labeled so that both thetransmitter and receiver simply need to notify the other which table isto be used without actually having to transmit the information again.For example, the stored BAT may be numbered. The transmitter or receiversimply needs to tell the other transceiver which BAT table number is tobe used for subsequent transmission. Either the receiver or thetransmitter can initiate the FSRA protocol.

Receiver-Initiated FSRA

The receiver-initiated FSRA protocol involves the following steps:

1. The receiver determines that the data rate should be increased ordecreased.

2. If a stored BAT matches the new channel and/or application conditionthe receiver proceeds to step 3. If there is no stored BAT that matchesthe condition, an NSRA is initiated (as described above).

3. The receiver sends a message to the transmitter specifying whichstored BAT is to be used for transmission based on the new channeland/or application condition. This corresponds to an “FSRA Request” bythe receiver.

4. The transmitter receives the “FSRA Request”.

5. The transmitter uses an inverted sync symbol as a flag to signal thereceiver that the requested stored BAT will be used for transmission.The stored BAT is used for transmission on the first frame, or a finitenumber of frames, following the inverted sync symbol. The inverted syncsignal corresponds to a rate adaptation “SRA Go” message sent by thetransmitter.

6. The receiver detects the inverted sync symbol (“SRA Go”) and the newBAT is used for reception on the first frame, or a finite number offrames, following the inverted sync symbol.

FIG. 6 shows a flow chart 600 depicting an embodiment of a process inwhich a Fast Seamless Rate Adaptive (FSRA) transmission bit rate changeis initiated by a receiver according to the principles of the invention.In FIG. 6, the steps described in action boxes 610 through 660correspond to the preceding discussion.

Transmitter-initiated FSRA

The transmitter-initiated FSRA protocol involves the following steps:

1. The transmitter determines that the data rate should be increased ordecreased.

2. If a stored BAT matches the new channel or/and application condition,the transmitter proceeds to step 3. If there are no stored BAT thatmatches the condition then an NSRA is initiated (as described above).

3. The transmitter sends a message to the receiver specifying whichstored BAT is to be used for transmission, based on the new channeland/or application condition. This corresponds to an “FSRA Request” bythe transmitter.

4. The receiver receives the “FSRA Request”.

5. The receiver sends back to the transmitter the “FSRA Grant” messageto grant the “FSRA request”.

6. The transmitter receives the “FSRA Grant”.

7. The transmitter uses an inverted sync symbol as a flag to signal thereceiver that the requested stored BAT will be used for transmission.The specified stored BAT is used for transmission on the first frame, ora finite number of frames, following the inverted sync symbol. Theinverted sync signal corresponds to a rate adaptation “SRA Go” messagesent by the transmitter.

8. The receiver detects the inverted sync symbol (“SRA Go”) and the newBAT is used for reception on the first frame, or a finite number offrames, following the inverted sync symbol.

FIG. 7 shows a flow chart 700 depicting an embodiment of a process inwhich a Fast Seamless Rate Adaptive (FSRA) transmission bit rate changeis initiated by a transmitter according to the principles of theinvention. In FIG. 7, the steps described in action boxes 710 through780 correspond to the preceding discussion.

The FSRA protocol can be completed very quickly. It requires only theexchange of two messages (FSRA grant and FSRA Request) and an invertedsync symbol. FSRA is faster than NSRA because the BAT is stored and neednot be exchanged. As in the NSRA protocol, the FSRA protocol is alsovery robust in noisy environments since it uses inverted sync symbolsfor the “SRA Go”.

Use of SRA Protocols for Power Management (Entering and Exiting LowPower Modes)

Full power mode is used during normal operations of the transceiver. Lowpower transmission modes are often used in transceivers in order toconserve power in cases when data does not need to be transmitted overthe line. Many modems have low power modes or “sleep” modes that enablea transceiver to operate at a significantly lower power level when thetransmission requirements are reduced. Many modems also have protocolsthat enable them to enter and exit these low power modes very quickly sothat the user is not negatively effected by the modem's transition intothe low power mode state. The SRA protocols provided of the inventionare used to enter and exit from low power modes in a very fast andseamless manner.

There are two basic types of low power mode (LPM):

Low Data Rate LPM

This is low power mode with a very low data rate (e.g. 32 kbps). Only afew of the subchannels are active. The data connection is maintained.The pilot tone may also be transmitted in order to maintain loop timing.

Zero Data Rate LPM

This is a low power mode with an effectively 0 kbps data rate, i.e., nosubchannels are modulating data. A data connection is not maintained.The pilot tone may also be transmitted in this case in order to maintainloop timing.

In both the Low Data Rate LPM and the Zero Data Rate LPM, the syncsymbol, which is sent in normal full power mode every 69 symbols, may beon or off. If the sync symbol is still transmitted during the low powermode, the receiver can use the sync symbol to monitor for channelchanges and other fluctuations on the line. However transmission of thesync symbol every 69 symbols can cause non-stationary crosstalk andcould be detrimental to other signals on the same telephone wire or inthe same wire bundle. If the sync symbol is not transmitted during lowpower mode, there is no non-stationary crosstalk on the telephone wireor the wire bundle. However, in this-case the receiver is not able tomonitor the channel with the sync symbol

Entering Low Power Mode Using FSRA

1. Receiver-initiated Transition Into Low Power Mode

The receiver initiates the transition to low power mode using thereceiver-initiated FSRA protocol. A receiver initiating the transitionto low power mode uses a stored BAT corresponding to the low power mode.The stored BAT table for the low power mode may enable either a Low DataRate LPM or a Zero Data Rate LPM- The low power mode BAT can bepredefined by the system or can be exchanged and stored using the NSRAprocess. In either case the receiver uses the receiver-initiated FSRAprotocol to designate the low power mode BAT and synchronously switch tousing that BAT for transmission.

2. Transmitter-initiated Transition Into Low Power Mode

There are two ways the transmitter can use the transmitter-initiatedFSRA protocol to enter into the low power mode. In one embodiment, thetransmitter can use the entire transmitter-initiated FSRA process andrequest the transition. As in the case of receiver-initiated transitioninto low power mode, transmitter initiating the transition to low powermode uses a stored BAT for the low power mode. The stored BAT table forthe low power mode can enable either a Low Data Rate LPM or a Zero DataRate LPM. The low power mode BAT can be predefined by the system or canbe exchanged and stored using the NSRA process. In either case thetransmitter uses the transmitter-initiated FSRA protocol to designatethe low power mode BAT and synchronously switches to the low power modeusing that BAT for transmission.

In a second embodiment, the transmitter can transition directly to step7 of the transmitter initiated FSRA protocol described above, and sendthe inverted sync symbol to indicate transition into the low power mode.The receiver detects the inverted sync and transitions to the low powermode. In this case, since an FSRA request has not been sent by thetransmitter, the receiver recognizes that an inverted sync symbolreceived without a FSRA request transmitted indicates that thetransmitter is switching to low power mode. The low power mode BAT is(predefined by the system) or is identified and stored previously sothat both the transmitter and the receiver use the BAT. In analternative second embodiment, in step 7 the transmitter sends adifferent signal that is predefined by the transmitter and the receiverto be the signal used for transition into low power mode without an“FSRA request.” For example, the transmitter may send a sync symbol with45 degree phase rotation, rather than the inverted (180 degree) syncsymbol. A sync symbol with a 45 degree phase rotation indicates that thetransmitter is transitioning into low power mode using the stored BATassociated with the low power mode on the first frame, or a finitenumber of frames, following the sync symbol with a 45 degree rotation.

The transmitter-initiated entry into low power mode as defined in thesecond embodiment has the advantage that it does not require the reversechannel to make the transition The reverse channel is defined as thecommunications channel in the opposite direction, i.e., here, thecommunications channel used to send the FSRA messages from the receiverto the transmitter. This is advantageous because the reverse channel mayalready be in low power mode with no data connection. If there is nodata ready to be sent the transmitter can simply transition to low powermode. This is an important power savings technique since the transmitterconsumes a large portion of the power, as it is required to send thesignal down the line. Transmitter-initiated transition into low powermodes is also useful in “soft modem” (PC host based) implementations. Ina soft modem implementation, the host processor is performing the modemtransceiver functions and many other PC applications at the same time.If the host processor must perform another task that does not allow itto run the ADSL transmitter, the processor can quickly transition thetransmitter to the low power mode by sending the inverted sync symbol,or the sync symbol with 45 degree rotation. After this the hostprocessor resources can be consumed by the other task. The ADSLtransmitter sends no signal (0 kbps) onto the line.

The transmitter-initiated and receiver-initiated protocols describedabove enable the communication system to enter a low power mode in eachdirection (upstream and downstream) separately or in both directionstogether. The cases described above each focus on one direction Theprotocols can be combined to accomplish transition in both directions atthe same time. As an example, assume that the customer premisetransceiver (CPT) is designed to enter into a low power mode in responseto a PC that is also entering a similar state. The CPT first usesreceiver-initiated low power mode transition to put the downstream (COto CPT direction) into low power mode. Afterwards the CPT uses thetransmitter-initiated low power mode transition to put the upstream (CPTto CO direction) into low power mode

Exiting Low Power Mode

1. Receiver-initiated Exit From Power Mode

According to the SRA protocols, there are two embodiments the receivercan use to exit the low power mode. In the first embodiment,receiver-initiated exit from low power mode can be accomplished usingthe receiver initiated NSRA or FSRA protocol if the low power mode stillhas at least a slow data connection in the reverse direction (Low datarate LPM). This is necessary because the receiver must be capable ofsending the SRA request back to the transmitter along with the BAT to beused. If the transmitter has not turned off the sync symbol in low powermode the NSRA or FSRA protocols would be used as described above. If thetransmitter sync symbol is turned off while in low power mode, the “SRAGo” is sent by the transmitter by turning the sync symbol back on. Thereceiver detects the presence of the sync symbol (with or withoutinversion) as a flag to synchronize the change in data rate.

In a second embodiment, there is no data connection in the reversedirection (Zero Data Rate LPM). The receiver initiates an exit by firstcompleting a “transmitter-initiated exit from low power mode’ (describedbelow) in the reverse direction. This enables the data connection in thereverse direction. The receiver uses the receiver initiated NSPA or FSRAprotocol to exit from low power mode in it's own direction. As describedabove, if the transmitter sync symbol is turned off while in low powermode, the “SRA Go” is sent by the transmitter by tuning the sync symbolback on. The receiver detects the presence of the sync symbol (with orwithout inversion) as a flag to synchronize the change in data rate.

2. Transmitter-initiated Exit From Low Power Mode

According to the SRA protocols, there are two embodiments thetransmitter can use to exit from low power mode. In the firstembodiment, the transmitter uses the entire transmitter initiated FSRAor NSRA process and requests the transition. This requires that there isa data connection in both directions (Low data rate LPM) so the protocolmessages can be exchanged. As in the receiver-initiated exit from lowpower mode, if the transmitter has not turned off the sync symbol in lowpower mode the NSRA or FSRA protocols would be used as described above.If the transmitter had turned the sync symbol off while in low powermode, then the “SRA Go” is sent by the transmitter by turning the syncsymbol back on. The receiver detects the presence of the sync symbol(with or without inversion) as a flag to synchronize the change in datarate.

In the second embodiment, the transmitter can exit the low power mode bytransitioning directly to step 7 of the transmitter initiated FSRAprotocol. The transmitter sends the inverted sync symbol to indicatetransition out of the low power mode. This requires that a sync symbolbe sent during the low power mode. This protocol does not require a lowdata rate LPM. The receiver detects the inverted sync and exits the lowpower mode. The receiver is designed to recognize that an inverted syncsymbol received without a FSRA request indicates the transmitter isexiting from low power mode. The full power mode BAT is identified andstored previously in the connection so that both the transmitter and thereceiver have the BAT. For example, the BAT to be used upon exiting alow power mode can be defined by the system to default to the BAT of thelast full power connection. Alternatively, the transmitter can send adifferent signal that is predefined by the transmitter and the receiverto be the signal used for transition out of low power mode without an“FSRA request”. For example, the transmitter can send a sync symbol with45 degree phase rotation, rather than the inverted (180 degree) syncsymbol. When the receiver detects the sync symbol with a 45 degree phaserotation, the receiver recognizes that the transmitter is transitioningout of low power mode using the stored BAT associated with the fullpower mode on the first frame, or a finite number of frames, followingthe sync symbol with a 45 degree rotation. If the transmitter had turnedthe sync symbol off while in low power mode, then the “SRA Go” is sentby the transmitter by turning the sync symbol back on. The receiverdetects the presence of the sync symbol (with or without a phase shift)as a flag to synchronize the change in data rate.

Although throughout this description the BAT is defined to be a tablethat specifies the number of bits allocated to each subchannel, the BATcan also contain other parameters associated with allocating bits tosubchannels in a multicarrier system. An example of an additionalparameter is the Fine Gain per subchannel as defined in the ANSI and ITUstandards. In this case, when the BAT is exchanged during the NSRAprotocol or the BAT is stored during the FSRA protocol, the BAT alsocontains the Fine Gain value for each subchannel.

The seamless rate adaptive system and associated protocols also appliesto DMT systems that implement dual (or multiple) latency paths. A duallatency system is defined in the ITU and ANSI standards as a DMT systemthat supports two data streams with different latency specifications inthe Framer/FEC block. FIG. 3 shows a standard ADSL DMT system 300 thatimplements dual latency, as an example of a system having a plurality oflatencies. The system 300 includes three layers: the Modulation layer310, the Framer/FEC layer 320, and the ATM TC layer 340, which aresimilar but not identical to the three layers described above inrelation to FIG. 1.

The Modulation layer 310 provides functionality associated with DMTmodulation. The DMT modulation is implemented using a Inverse DiscreteFourier Transform (IDFT) 112. The IDFT 112 modulates bits from the dualinput Quadrature Amplitude Modulation (QAM) 314 encoder into themulticarrier subchannels. The operation of the Modulation layer 310 isanalogous to that of Modulation layer 110 of FIG. 1, with the differencethat the Modulation layer 310 has multiple inputs, rather than only oneinput.

The Frame/FEC layer 320 shown in FIG. 3 has two paths. This layercontains a first path that includes the same blocks as in the Frame/FEClayer 120 of FIG. 1, namely the Interleaving (INT) block 122, theForward Error Correction (FEC) block 124, the scrambler (SCR) block 126,the Cyclic Redundancy Check (CRC) block 128 and the ADSL Framer block130. The layer further contains a second path that includes a second oneof each of the Forward Error Correction (FEC) block 124′, the scrambler(SCR) block 126′, the Cyclic Redundancy Check (CRC) block 128′ and theADSL Framer block 130′. The Frame/FEC layer 320 provides functionalityassociated with preparing a stream of bits for modulation,

The new lower path through the Framer/FEC layer 320 has a differentamount of latency than the original upper path corresponding to FIG. 1,because the lower path does not perform interleaving on the data stream.Dual latency is used to send different application bit streams withdifferent latency requirements through the ADSL DMT modem. As anexample, an application that can tolerate high latency (e.g., video ondemand) may be sent through the upper high latency path withinterleaving whereas the an application with low latency requirements(e.g., voice) may be sent through the lower low latency path withoutinterleaving.

The ATM TC layer 340 includes an ATM TC block 342 having multiple inputsand multiple outputs that transforms bits and bytes in cells into framesfor each path.

The seamless rate adaptation system and method of the present inventionapplies to a system with dual latency, or even multiple latency, aswell. In the case of dual latency, the FEC and interleaving parametersfor both paths are decoupled from the DMT symbol size. The BAT contains,in addition to the number of bits allocated to each subchannel, the datarate for each latency path in the form of bits per DMT symbol. Whenseamless rate adaptations are performed using the FSRA and NSRAprotocols the BAT also indicates the data rate for each latency path.For example, if the dual latency system runs with 1.536 Mbps on theinterleaved path (high latency upper path) and 256 kbps in thenon-interleaved path (low latency lower path) and an SRA is initiated,then the SRA protocol specifies the new BAT containing the number ofbits per subchannel and also the new data rate for each latency path. Ata 4 kHz DMT symbol rate, a system running at 1.536 Mbps+256 kbps=1.792Mbps 179200014000=448 total bits per symbol. The BAT specifies that1536000/4000=384 bits per symbol are allocated to the interleaved pathand 256000/4000=64 bits per symbol are allocated to the non-interleavedpath. In the example, when an SRA is performed, the new data rate forthe interleaved path can be 1.048 Mbps (1048000/4000=262 bits persymbol) and the new data rate for the non-interleaved path can be 128kbps (128000/4000=32 bits per DMT symbol), resulting in a totalthroughput rate of 1.176 kbps (or 294 total bits per DMT symbol). TheNSRA and FSRA protocols combined with the framing method specifiedherein complete this data rate change in both latency paths in aseamless manner. It is also possible to not change the data rate on bothlatency paths. For example one may want to keep the 256 kbps low latencypath at a constant data rate because it is carrying voice data (multipletelephone calls) that can not operate at a lower rate, whereas the 1.536Mbps path may be carrying internet access data that can tolerate a ratechange. In this example, during the SRA the data rate of the low latencypath is kept constant at 256 kbps whereas the data rate of the highlatency path changes.

While the invention has been disclosed in connection to ADSL systems itcan also be applied to any system that utilizes multicarrier modulation.In general this invention applies to any system in which differentnumbers of bits are modulated on the carriers.

While the invention has been particularly shown and described withreference to specific preferred embodiments, it should be understood bythose skilled in the art that various changes in form and detail may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

1. In a multicarrier transmission system including a transmitter and areceiver, the transmitter and receiver using a first bit allocationtable to transmit a plurality of codewords at a first transmission bitrate in a first power mode, the plurality of code words having aspecified codeword size and including a specified number of parity bitsfor forward error correction, and a specified interleaving parameter forinterleaving the plurality of codewords, the system seamlessly enteringa second power mode comprising: means for storing a second bitallocation table at the receiver and at the transmitter for transmittingcodewords at a second transmission bit rate in the second power mode;means for synchronizing use of the second bit allocation table betweenthe transmitter and receiver; and means for transitioning the secondpower mode by using the second bit allocation table to transmitcodewords, wherein the specified interleaving parameter, the specifiedcodeword size, and the specified number of parity bits for forward errorcorrection used to transmit codewords in the first power mode are alsoused to transmit codewords in the second power mode to achieve aseamless change in power mode.
 2. The system of claim 1, wherein thesynchronizing includes sending a flag signal.
 3. The system of claim 2,wherein the flag signal is a predefined signal.
 4. The system of claim3, wherein the predefined signal is a sync symbol with a predefinedphase shift.
 5. The system of claim 3, wherein the predefined signal isan inverted sync symbol.
 6. The system of claim 2, wherein thetransmitter transmits the flag signal to the receiver.
 7. The system ofclaim 2, wherein the receiver transmits the flag signal to thetransmitter.
 8. The system of claim 1, wherein the second power mode isa low power mode.
 9. The system of claim 8, further comprising means forallocating zero bits to carrier signals to achieve a transmission bitrate of approximately zero kilobits per second in the low power mode.10. The system of claim 8, further comprising means for transmitting apilot tone for timing recovery when operating in the low power mode. 11.The system of claim 8, further comprising means for periodicallytransmitting a sync symbol when operating in the low power mode.
 12. Thesystem of claim 2, further comprising means for using the first bitallocation table for transmitting a plurality of DMT symbols in thefirst power mode and switching to the second bit allocation table fortransmitting the plurality of the DMT symbols in the second power mode,wherein the second bit allocation table is used for transmissionstarting with a predetermined one of the DMT symbols that follows thetransmission of the flag signal.
 13. The system of claim 12, wherein thepredetermined DMT symbol is the first DMT symbol that follows thetransmission of the flag signal.
 14. The system of claim 1, wherein thesecond power mode is a full power mode.
 15. The system of claim 1,wherein the first power mode is a full power mode, and the second powermode is a low power mode.
 16. The system of claim 1, wherein the firstpower mode is a low power mode, and the second power mode is a fullpower mode.